1. Field of the Invention
This invention relates to the production of beta-alumina.
2. Prior Art
Beta-alumina is a solid oxide ceramic, which conducts sodium ions and has a negligible electronic conductivity. It has a composition by weight of 5 to 10% sodium oxide, the balance being alpha-alumina with small additions of magnesium oxide and/or lithium oxide, which tend to enhance the ionic conductivity of the beta-alumina. This material is used in a variety of electrical energy and material conversion devices, for example as the electrolyte and separator in a sodium sulphur cell.
Articles of beta-alumina may be produced by sintering shapes formed of powdered material. These shapes may be produced by a number of different production routes. For example, the powder may be formed from the constituent oxides or it may be powdered beta-alumina or it may be a mixture of the constituent oxides and beta-alumina. The green shapes may, for example, be produced by dry vibro-energy milling together of the powdered material, e.g. alpha-alumina, sodium aluminate and magnesia, and then forming a required shape, e.g. a tube, by isostatic pressing. These shapes may then be sintered in a pass-through furnace as described for example, in U.S. Pat. Nos. 3,903,225 and 3,950,463. The use of dry milling as a method of mixing and comminuting the constituent materials is satisfactory from the point of view of producing a high sintered density in the final article, but this powder has relatively poor flow properties. These poor flow properties are quite acceptable in the production of small numbers of articles but, for a high rate of production, such as may be required with automated production techniques, a free-flowing powder with a reproducible particle size distribution is preferable in order to ensure the production of green shapes of uniform density by isostatic pressing. Dry vibro-energy milling is a batch process and, for commercial production of beta-alumina articles, it would also be preferable to have a technique by which free-flowing powders can be made on a continuous basis. The present invention employs a spray drying technique of an aqueous slurry to make a free-flowing powder. Spray drying equipment is commercially available, which can produce powder on a continuous basis at an economic cost. The spray drying process may employ a stable slurry of powder in water or in an aqueous solution of one or more of the other constituents; a fine spray of slurry droplets is produced by atomisation, and these are then dried in a flow of hot gas, usually air. The large surface area of slurry exposed to the hot air results in rapid drying of the droplets to form spherical aggregates. The powder produced by the drying of the droplets may be separated from the air using a cyclone. It has been found readily possible, by spray drying, to produce spherical aggregates of beta-alumina precursor powder having excellent flow properties. Moreover the powder may be made free from second phase impurities and can readily be made with a high degree of chemical homogeneity.
The semi-annual report dated January 1976 of Ford Motor Co. Dearborn, Mich., to RANN Division of Advanced Energy Research and Technology, Washington, D.C. under Contract No. NSF-C805 describes tests to produce beta-alumina articles by a process including spray-drying of the powdered oxide material with a number of organic binders, but the report states that the dried powder produced thereby could not be pressed to form thin-walled tubes.
We have, moreover, found that, if an aqueous slurry is formed of mixed oxide material, for example sodium oxide, aluminium oxide and magnesium oxide, or from powdered beta-alumina containing magnesium oxide, such as have been used heretofore in the production of articles of beta-alumina, the density of the sintered beta-alumina produced from the dried powder is low compared with that produced by the heretofore known techniques employing dry milled powder.